1. Field of the Invention
The invention relates to aircraft radio based area navigation (RNAV) and particularly with regard to OMEGA and VOR/DME RNAV aids.
2. Description of the Prior Art
VOR/DME radio navigation aids are utilized to provide latitude and longitude positional data to aircraft equipped with suitable RNAV receivers. A VOR/DME station utilizes a conventional VOR transmission system to provide bearing data to the aircraft with regard to the station location as well as a standard DME system to provide distance data to the aircraft with regard to the station. Analog and/or digital equipment on board the aircraft converts the bearing and distance data with respect to the fixed location of the station into aircraft latitude and longitude positional data in a well known manner.
The positional data provided by the VOR/DME RNAV aid is accurate when the aircraft is relatively close to the VOR/DME station but the accuracy deteriorates at substantial distances from the station. Such systems provide accuracies of several tenths of a mile within approximately ten miles of a station but have an error of from five to ten miles at distances of 100 to 200 miles from the facility. An error no greater than approximately two miles is desired throughout the enroute flight of the aircraft to permit reduction of air route lane widths.
Previous attempts at enhancing enroute accuracy have involved the use of dual separated DME systems. Although more accurate than the VOR/DME system at significant distances from the stations, the DME/DME system requires two complete DME receivers (A DME receiver being more complex than a VOR receiver) as well as a significantly more complex way point or leg definition based on the two DME stations which provide range vectors with a significant angle with respect to each other as compared to the substantially simpler VOR/DME navigation system. Alternatively, inertial navigation equipment with radio update when the aircraft is close to a station has been used in navigation systems but inertial navigation equipment is extremely expensive compared to the simpler radio systems.
As is known, OMEGA is a low frequency hyperbolic navigation system providing latitude and longitude positional data throughout the world. The OMEGA system achieves one to two mile accuracy but OMEGA receivers require elaborate equipment to correct for propagation effects in order to achieve this accuracy, such propagation effects typically being of a slowly varying diurnal nature.